1. Field of the Invention
The present invention relates to electrical heating cables that use positive temperature coefficient polymeric materials as self-regulating heating elements.
2. Description of the Prior Art
Electrically conductive thermoplastic heaters that exhibit a positive temperature coefficient (PTC) characteristic are well known in the art. These heaters generally used conductive polymers as the heat generating source. Other well known PTC heaters are those using doped barium titanate chips or disks rather than a conductive polymeric PTC composition.
In heaters of both types mentioned above, the temperature sensitive material of the heating element, either a conductive polymeric PTC composition (hereinafter referred to as PTC composition) or a doped barium titanate chip (hereinafter referred to as PTC chip), has a temperature limit essentially equal to the desired self-limiting temperature of the heating cable and undergoes an increase in temperature coefficient of resistance when this limit is reached, so that the resistance of such heating element increases greatly. The current flowing substantially decreases in response to the increased resistance, limiting the power output from the cable to thereby prevent overheating of the heating cable. The point at which this sharp rise in resistance occurs in the PTC chip heater is termed the Curie point or switching temperature and is fixed by the dopant material. The switching temperature of the PTC composition heater is generally determined by the degree of crystallinity of the polymer and the polymer melt point. It may be a rather well defined temperature, or depending upon the polymer, it may take place over a temperature range and be somewhat less precise.
Generally, the conductive thermoplastic material used to make PTC composition heaters is produced by compounding carbon black particles and a crystalline thermoplastic polymer in a suitable blender. Typically, the blended material is extruded upon two or more spaced apart conventional, round, stranded bus wires, to form a heater matrix core, as shown in FIG. 1. A variety of other processing operations may take place following the extrusion process, such as the application of an electrically insulating jacket, annealing, cross-linking, etc. Heating cables are often supplied to the end user with an outer braided metallic jacket of copper, tinned copper or stainless steel which is applied over the primary electrical insulation covering the PTC composition heater. Generally, a protective overjacket of polymeric material is then extruded over the braid, especially if the braid is copper or tinned copper to prevent corrosion of the metallic braid.
Typically, the conductive compositions of polymer and carbon contain from about 4% to about 30% by weight of electrically conductive carbon black. Ideally, the conductive carbon black is uniformly dispersed throughout the matrix.
A practical description of how a PTC composition heating cable such as the one shown in FIG. 1 works is as follows: The bus wires are connected to an electrical power source and current flows between the buses through the conductive matrix. When the matrix is cool and dense the carbon particles are in contact, forming an electrically conductive network. When the matrix begins to heat up, the matrix expands and the conductive carbon network begins to break contact, disrupting the current flow and reducing the heating energy of the cable. As more of the carbon network is disrupted, the temperature drops, contracting the matrix, resulting in greater current flow and heat production. Eventually the cable reaches a self-regulated state reacting to the environment. Each point along the conductive matrix will adjust to its local temperature environment independently of the adjacent portion of the core material.
It has been recognized that by adjusting the heat transfer rate from a resistive heating element, the surface temperature can be changed. In a heater of a fixed resistance, of either a series of parallel configuration, the heater sheath or surface temperature is not at a constant temperature. The cable or heater sheath temperature varies according to the amount of power the heater produces, the heat transfer rate from the heater to the pipe or equipment, the heat transfer or surface area of the heater and the process temperature or temperature of piping to which the cable is applied. At a constant voltage, the power output of a "fixed resistance" heater will not vary, but the sheath temperature of the heater can vary greatly depending upon the overall heat transfer rate from the heater to the pipe or equipment surface. Different methods of attachment of heaters to a pipe with resulting differing heat transfer coefficients result in sheath temperatures of the fixed resistance heaters varying from the highest sheath temperature when only strapped to a pipe at regular intervals, to a lower temperature when covered with wide aluminum tape running parallel over the heater and holding the heater to the pipe, to an even lower temperature when attached to the pipe with a heat transfer compound.
In a PTC composition heater, there is no fixed energy output since the resistance is a function of the temperature of the conductive matrix. A higher or lower energy output can be obtained by changing the heat transfer rate from the conductive matrix to its surrounding environment.
When voltage is applied to a PTC composition heater, it will generate energy. If the heat transfer rate from the conductive matrix is low, then the heater will self-heat rather quickly and reach its switching temperature at a lower total output than would occur if a good means of heat dissipation were provided. Unlike a "fixed resistance" heater, an increase in supply voltage has very little effect on the output of a PTC composition heater.
A great number of PTC composition heater assemblies exist in the prior art. A number of these heaters were developed to provide low inrush current or to improve the power output of the PTC composition heaters. Generally, the assemblies have all been based on a layered concept which utilizes PTC composition materials and constant wattage (CW) or relatively constant wattage (RCW) materials in a layered or alternate configuration.
As previously stated, it was known that a reduction in sheath temperatures could be achieved by the application of heat transfer aids to the external surface of resistive heating cables. However, the heat transfer capabilities of heating cables were still limited, even with the use of external transfer improvements, because of internal heat transfer limitations. Better internal heat transfer was necessary to improve the heating characteristics of the cable.
Although it was known that flat electrodes, generally formed by a metallic mesh, grid or thin sheet, could be used to supply electrical power to the PTC composition material as shown in U.S. Pat. No. 4,330,703, the assemblies utilizing these prior flat electrodes still had low internal heat transfer properties because the electrodes were thin and had poor heat thermal transfer characteristics. Further, the heat producing materials in the cables were generally a combination of PTC compositions and CW materials, not single PTC compositions, resulting in increased costs. Additionally, the prior designs utilizing flat electrodes did not provide for easily embedding the electrodes in the PTC composition in an extrusion process, a low cost manufacturing process.